The present invention relates to particles, such as carbon black, having an attached stable free radical which permits the formation of blocked radical sources. The present invention further relates to methods of preparing and using such modified particles.
The role of carbon black as a thermal stabilizer in polymeric systems is discussed by W. L. Hawkins, R. H. Hansen, W. Matreyek, F. H. Winslow; J. Applied Polymer Science, vol. 1, pages 37-42, 1959; J. T. Cruver, K. W. Rollmann: J. Applied Polymer Science, vol. 8, pages 1169-83, 1964, and by G. Ivan, M. Giurgina: Revue Roumaine de Chemie, vol. 29, number 8, pages 639-646, 1984. In each discussion the mechanism is through phenolic and quinone oxygen groups on the carbon black surface behaving either as radical traps or peroxide decomposers. One skilled in the art, however, would consider these sites unavailable as initiating sites for polymerization processes.
Belmont et al. (J. A. Belmont, J. M. Funt: International Rubber Conference, Essen, Germany, June 24-27 1991) identified the presence of peroxide groups, typically in the range 0.1 to 0.4 micromoles/sq meter, on the carbon black surface. However, the majority (greater than 80%) of the peroxide groups are thermally stable to heat treatment at 200.degree. C. and hence cannot be considered to be potential initiating sites for radical polymerization.
Tsubokawa et al. (K. Fujiki, N. Tsubokawa, Y. Sone: Polymer J., vol. 22, number 8, pages 661-70, 1990, and N. Tsubokawa: Prog. Polymer Science, vol. 17, pages 417-70, 1992) discuss growing polymers from an activated carbon black surface by first attaching a reactive group via the oxygen groups on the carbon black surface. Typical examples include the use of glycidyl methacrylate where the glycidyl group reacts with phenolic hydroxyl groups on the carbon black surface providing a vinyl functionality; the reaction of 4,4' azo bis-(4-cyanovaleric acid) whereby the isocyanate group reacts with phenolic hydroxyl groups and subsequent heating decomposes the azo group to generate an alkyl radical; and the reaction of the phenolic hydroxyl groups with butyl lithium which can then be used as an initiation site for anionic polymerization.
All of these approaches require the presence of oxygen groups on the carbon black surface. Since the level of reactive hydroxyl and carboxylic acid groups, relative to the above approaches, present on a typical furnace or thermal carbon black is typically 0.01 to 0.1 micromoles/sq meter, the number of potential initiation sites is quite low.
Further, subsequent polymerization from these activated sites will most likely occur in the normal manner for free radical polymerization with the chains being irreversible terminated by chain combination reactions, combination with unreacted oxygen groups on the carbon black surface, and/or the addition of chain stoppers. In all cases the polymerization cannot be reinitiated. Accordingly, there is a need to provide particles with attached stable free radicals which overcome the above-described limitations.